This invention relates to ion sources for use in a vacuum environment. The ion source disclosed is of the type where a propellant, reduced to the gaseous state, is ionized by electron bombardment and accelerated to high energy using an electrodeless acceleration channel. The energetic beam from this type of ion source can be used for a variety of purposes which include surface modification, cleaning, deposition and etching of integrated circuits, solar cells, architectural glass, consumer packaging, and machine tooling. Similarly, the energetic beam from this type of ion source can also be used to provide thrust in space applications where the ion source serves as a propulsion device for spacecraft attitude and orbit maintenance and repositioning functions.
Ion sources for use in a vacuum environment are known. Most ion sources that have been developed for either ground based ion beam processing applications or spacecraft propulsion applications have used electrostatic ion accelerator systems utilizing two or three accelerating electrodes. These electrodes, or grids as they are often called, can contain thousands of precisely aligned apertures, each producing a tiny beamlet all of which combine to give the resulting ion source total ion beam. Ion sources have been developed which produce an energetic ion beam using an electrodeless ion acceleration principle. Attention is directed to U.S. Pat. No. 4,541,890 to J. J. Cuomo et al. and to U.S. Pat. No. 4,862,032 to H. R. Kaufman et al. These prior art designs have low utilization of the propellant gas, produce relatively low energy ion beams, are very sensitive to background vacuum pressure conditions, and depend on a cathode in the ion beam for successful ion acceleration which severely limits cathode lifetime.
A high energy ion beam electrodeless ion source has been developed for space propulsion applications in Russia. A review and reference source for these so called Hall current, or, Stationary Plasma Thrusters (SPT's), is provided in Jet Propulsion Laboratory Publication 92-4 by J. R. Brophy. A Hall current, or SPT, thruster has also been reported in work by K. Komurasaki et al. from Japan. The basic ion acceleration mechanism for the SPT is well known and attention is directed to U.S. Pat. No. 3,309,873 to G. L. Cann. However, the prior art of this type of electrodeless ion source uses either electromagnets or hollow cathodes which are positioned outside the ion source body. This makes for a relatively large volume device which is detrimental for the limited space inside ground based plasma processing vacuum chambers, and for integration to volume constrained spacecraft. Moreover, the prior art Hall current, or SPT ion sources rely on the plasma discharge current passing through the electromagnet solenoids which severely limit their power throttling range due to a rapid drop off in gas ionization efficiency as the accelerating channel magnetic field is necessarily reduced. Similarly, the prior art Hall current, or SPT ion sources experience large erosion of internal source components and significant hollow cathode erosion.